Fluxes are commonly reacted with surface oxides, such as tin oxide, during the reflow process to generate metal surfaces which subsequently intermix forming a solder joint. Problems in fluxing and soldering can arise in which inadequate amounts of flux or excessive thicknesses of tin oxide are present either or both of which can inhibit wetting and intermixing and result in the formation of non wetted, electrically open solder joints. In addition, where excess flux may be incompletely consumed in the reflow process. The remaining residues will interfere with the efficiency of subsequent process steps such as chip underfill.
Routinely, tin oxide is controlled by multiple reflows of solder in the presence of flux which subsequently must be removed, either by aqueous, organic solvent, or gaseous cleaning. The newly-cleaned surfaces are now subject to reoxidation by exposure to ambient air, thus the thickness of tin oxide layer may vary unacceptably in an uncontrolled manner.
Currently a variety of flux types, both those that do not require a separate cleaning step, no clean, and types that are removed by aqueous, or organic solvent washing are employed in the electronics industry. These fluxes introduce residues onto laminate, solder joint and chip surfaces which can inhibit proper underfill flow required for subsequent mechanical stability of the solder joints.